Conventional golf balls generally have a cover structure that is formed by injection molding a material composed primarily of an ionomer resin over a core made chiefly of a diene rubber. In recent years, to improve the feel of the ball at impact and the scuff resistance on approach shots, golf balls having a cover composed primarily of a urethane material such as a thermoplastic polyurethane elastomer have become more common.
When a urethane material is used as the cover, a good feel on impact and a high scuff resistance can be obtained. However, compared with ionomer resins, adhesion to the diene rubber serving as the chief ingredient of the core may be poor and, when the ball is repeatedly struck, adherence to the core surface may worsen. Also, in golf balls having a plurality of cover layers, the adhesive properties between urethane and ionomer resin and the adhesion with neighboring cover layers are also important. When the adhesion between layers of the golf ball is poor, this may invite declines in various ball properties, such as the distance, spin rate on approach shots, feel on impact, durability to cracking and scuff resistance. Hence, there exists a desire to improve the adhesion between layers.
Art for improving adhesion between the layers of a golf ball has been disclosed in a number of patent publications, one of which, JP-A H5-68724, describes the inclusion of an oxazoline group-containing thermoplastic resin in an ionomer resin-based cover material. This patent publication mentions that when an oxazoline group-containing thermoplastic resin is mixed into an ionomer resin under applied heat, the oxazoline groups in the thermoplastic resin react with carboxyl groups in the ionomer resin, causing the oxazoline group-containing thermoplastic resin to graft onto the surface of the ionomer resin, thus forming a compatible mixed system, or “polymer alloy,” of the oxazoline group-containing thermoplastic resin microdispersed within the ionomer resin. Synergistic improvements in the physical properties of this polymer alloy enable the toughness of the ionomer resin to be further improved, as a result of which the golf ball has an increased durability.
However, the golf ball of the foregoing art includes an oxazoline group-containing thermoplastic resin in a resin composition composed primarily of an ionomer resin; it is not art relating to a resin composition for golf balls that is composed primarily of a polyurethane material and used for the purpose of obtaining a high scuff resistance.
Moreover, with regard to the golf ball resin compositions made up primarily of a urethane material such as a thermoplastic polyurethane elastomer that have hitherto been used, there also exists a desire to achieve an even higher scuff resistance while retaining, for example, a good ball flight performance, a good spin performance and a good feel on impact.